1. Field of the Invention
This invention relates to an exposure device, a processor and an exposing method for a substrate particularly having large unevenness in thickness as represented by a glass substrate used for a liquid crystal display or the like, a method of producing a thin film transistor, and a method of producing a display device.
2. Description of the Related Art
A glass substrate used for a semiconductor substrate, a liquid crystal display or the like is formed by patterning and stacking a plurality of materials such as semiconductor layers, insulation layers and the like. A technique of lithography is employed for the patterning.
According to the lithography, a photoresist is applied to a material to be processed to form a photoresist layer, an exposing pattern is formed on the resist layer (surface), the resist layer is developed, processing such as etching, deposition or the like is selectively performed on a portion obtained (left) by removing the resist layer of the non-developed portion, and a circuit, a transistor and the like are formed. A typical scheme of exposure is projection exposure such as lens projection exposure, mirror projection exposure, or the like. According to this exposure scheme, an image (exposure pattern) of an original plate (mask) is projected to the surface of the processed article to which the resist layer is applied, to form an image corresponding to the exposure pattern on the resist layer (by forming an image of the exposure pattern).
In the projection exposure, a focal distance of a projector, i.e., an image-formed surface formed by the projection exposure, is required to be equal to a distance from the projector to the exposed surface (resist surface).
In some cases, however, the resist layer surface, i.e. the exposed surface is microscopically wavy. The waviness on the resist surface layer is caused due to unevenness in thickness of the substrate on which the resist layer is formed, binding of the substrate, flatness of a stage on which the substrate is placed, and the like.
An amount of variation in the waviness on the resist layer surface needs to fall within a range in which a depth of focus (DOF) of an image-forming optical system (incorporated in the projector) can be maintained at a predetermined level. Needless to say, if the distance between the exposed surface and the projector is out of the range of the DOF, a light intensity distribution (air image) of an image-forming pattern is deformed and an expected resist pattern (in a flat or cross-sectional shape) cannot be obtained. In other words, portion A that is out of the range of the DOF is not resolved as illustrated in FIG. 21.
In general, a relationship between the resolution of the image on the resist layer and the DOF, in the exposure, is defined by the following formula:DOF=k·R2/λwhere λ represents a wavelength of a light source, R represents a line width (resolution), and k represents a factor of proportionality (value of about 1 according to the process).
At the exposing time, the exposed surface of the resist layer surface needs to fall within the range of the DOF. In other words, the exposed surface needs to be located at a position satisfying the following formula (1):DOF>“unevenness in thickness of substrate”+“waviness on surface of stage (substrate retaining portion)”+“accuracy of focusing (permissible value of deviation of the focal position of the image-forming optical system in the projector and the focal distance which is inherent to the image-forming optical system)”+“waviness caused by a processed layer”+“influence from aberration of the image-forming optical system”+“degree of freedom of the process”  (1)
If a thin-substrate a is placed on a stage S as illustrated in FIG. 19, the magnitude of waviness on the resist layer surface, i.e. the exposed surface, of the substrate a, is equal to a sum of “(the amount of unevenness in thickness of the substrate)+(the magnitude of waviness on the stage surface)”. They are entirely called “undulation T”.
Jpn. Pat. Appln. KOKAI Publication No. 2001-36088 discloses that such “undulation T” is cyclic, and that the “undulation T” may cause a problem when the microscopic pattern is exposed.
However, a large-scale exposure device, for example, an exposure device capable of forming a predetermined pattern on a glass substrate used for a large-scale liquid crystal display device (having a size of 20 (inch)×25 (inch), or a diagonal line longer than 32 inches) (or a glass substrate which is set on the exposure device) can hardly satisfy the conditions of formula (1) for the following reasons a) and b):    a) Recently, a high-definition image has been required to display the digital information. Thus, as high resolution is required (i.e., as the line width R is made smaller), the band in which the DOF is permissible becomes narrower.    b) Particularly, in a case of a liquid crystal display, a silicon wafer having good flatness (as used as a substrate of a semiconductor device) is not, but a glass substrate having a great unevenness in thickness (as compared with the flatness of the silicon wafer) is used. In the manufacturing process, as the glass substrate is selected to be so large that a plurality of glass substrates for single display devices can be formed thereon, the unevenness in thickness of the glass substrate is apparently increased on the entire area of the glass substrate set on the exposure device.
As a result, if the line width is to be smaller and the resolution is to be higher, the maximum value and the minimum value of the unevenness in thickness of the glass substrate go out of the DOF.
It is assumed here that, for example, if a pattern having a (minimum) line width R=1.0 μm is exposed as an exposure field which is 100 mm square, on a substrate for liquid crystal display having a size of 550×650 mm, the following values are employed:λ=0.365 μm (utilized wavelength), k=1.0At this time, DOF=k·R2/λ=2.7 μm
The exposure field needs to be sufficiently larger than the substrate size in order to reduce a time required for exposure of the overall substrate. In general, the exposure field which is about 100 mm square is employed, for a substrate for liquid crystal display which is larger than a size of about 550×650 mm.
The unevenness in thickness of the glass substrate for liquid crystal display is, in general, about 10 μm (Peak to Peak) to a width of 100 mm.
Therefore, the DOF is smaller than the unevenness in thickness of the substrate, and the formula (1) will not be satisfied even if the other terms are “0”. In other words, the image of the exposed mask (i.e., the light intensity distribution on the exposed surface) is blurred at a position where the image-formed surface and the exposed surface (i.e., the resist surface) greatly separate from each other due to the unevenness in thickness. This matter is also handled as a problem in Jpn. Pat. Appln. KOKAI Publication No. 2001-36088.
It has already been known that the unevenness in thickness of the glass substrate for liquid crystal display is substantially one-dimensional as illustrated in FIG. 23. In other words, the thickness is uneven in a direction x while it is even in a direction y in FIG. 23. As a typical producing method of the substrate glass, there are a fusion method and a float method. The conditions of the unevenness in thickness are derived from characteristics of the producing methods.
On the other hand, as the exposure scheme employed to produce a thin-film transistor (TFT) used for the liquid crystal display and the like, there are mainly “step-and-repeat” scheme and “step-and-scan” scheme. In the exposure device in either of the schemes, however, there is the only stage retaining the substrate for exposure, but necessity of a mechanism of flattening the wavy surface or a stage for measurement of the waviness is not considered.
In the step-and-repeat scheme, the unevenness on the exposed substrate surface is measured by an autofocus system and an optimum surface is determined by tilting or up-and-down movement, before performing static exposure of each shot on the substrate stage for exposure.
In a case where a mechanism of flattening the wavy surface is added to the substrate stage to perform the exposure of the step-and-repeat scheme, the waviness on the surface of the substrate placed on the substrate stage is measured before performing sequential operations of static exposure (or merely exposure operations). For this reason, a time passing until the end of exposure is longer and throughput is deteriorated.
In the step-and-scan scheme, c) the waviness on the surface of the substrate placed on the substrate stage is measured before scanning exposure, and d) simultaneously with the start of the scanning exposure, the scanning exposure is performed while determining the optimum exposed surface by tilting and/or up-and-down movement, on the basis of the measurement data.
At this time, the scanning exposure may be performed simultaneously with addition of focus control. Thus, in the step-and-scan scheme, the time to measure the waviness on the substrate surface to be exposed is included in the exposure time, the time passing until the end of exposure becomes longer, and the throughput is deteriorated similarly to the step-and-repeat scheme.
Incidentally, Jpn. Pat. Appln. KOKAI Publication No. 63-260129 discloses a method of measuring the waviness on the substrate surface and flattening the waviness by, for example, a plurality of two-dimensionally aligned piezoelectric elements, before conveying the substrate to be exposed onto the exposure stage. However, precise control means partially adjusting the height by two-dimensionally aligned piezoelectric elements are needed.